Scientists use scanning probe microscopes (SPMs) to reveal data about various properties of materials, such as gold or silicon, at very fine resolution, down to molecules and atoms of the materials. SPMs are a family of high magnification instruments that include Scanning Tunneling Microscopes (STMs), Atomic Force Microscopes (AFMs), Near Field Scanning Optical Microscopes (NSOMs), among others.
As an example, a typical STM may include a flat support upon which a material specimen is placed, and a small sharp probe suspended above the specimen by a piezoelectric tube. The probe has a tip so fine it is about the size of a single atom, and the piezoelectric tube can move the probe in step sizes smaller than the size of an atom. Voltage is applied to selected portions of the piezoelectric tube to expand or contract those portions so as to move the finely tipped probe vertically toward the specimen surface within a few diameters of an atom, and then scan over the specimen in a very precise lateral zig-zag pattern, side-to-side and up-and-down the specimen surface. The SPMs record physical interactions between the probe and the specimen surface as a function of the lateral and vertical position of the probe to create computer graphical images representative of the surface texture of the specimen. For example, voltage is applied across the STM probe and its electrically conductive specimen, to generate a tunneling current between the probe and specimen as the probe scans over the specimen. Then, command voltage to the vertical axis portion of the piezoelectric tube may be varied to maintain a constant tunneling current. Consequently, a digital image of the specimen surface may be recorded as variations in vertical probe position as measured by feedback voltage via the piezoelectric tube. This basic SPM technique is known as scanning probe topographic imaging.
In a more advanced SPM technique called scanning probe spectroscopy, spectroscopic analysis is typically conducted at a point location above the specimen surface. Spectroscopic analyses help extract more and different types of data from the surface than what is obtained from the basic scanning probe topographic imaging. For instance, spectroscopy may reveal data about local density of electronic states of the specimen, such as band gap qualities of a semiconductor. In a specific example of scanning tunneling spectroscopy, current may be measured as a function of variation in the voltage applied across a probe and specimen to yield a current-voltage curve, conductance-voltage curve, or the like. For this spectroscopic information to be relevant, the exact location where the spectroscopic data is collected must be accurately known.
In typical use, a topographic scan yields a topographic image, and then the probe is moved to a user-defined location within the topographic scan area above the specimen surface to conduct the spectroscopic analysis. A user may define a target location by using a computer mouse to move a pointer over a desired location on the previously acquired topographic image and clicking the mouse to select the desired location. The computer controller applies suitable voltage to the piezoelectric tube to move the probe to a position that corresponds with that location and the spectroscopic analysis is performed.
To improve the signal to noise ratio of spectroscopy, it is common to attempt many spectroscopic analyses at a target location and average the data together to yield an averaged spectroscopic analysis. But the problem with this approach is that the piezoelectric tube suffers from probe positional errors that may, for example, prevent the probe from actually arriving at and staying in the same exact target location on the specimen. In other words, probe positional errors that occur over the duration of several spectroscopic analyses will cause each spectroscopic analysis to be acquired at a different actual position. Thus, the averaged spectroscopic analysis data may not be truly representative of the spectroscopic characteristics of the specimen surface at the target location.
Such probe positional errors may include non-linearity, creep, hysteresis, and/or drift. Non-linearity means the movement of the piezoelectric elements does not respond linearly to the applied control signal. For example, application of a two volt signal to the piezoelectric element will not move the probe exactly twice the distance as application of a one volt signal. Creep means that the movement of the piezoelectric elements does not exactly follow the movement commanded by the controller. For example, each movement command of the probe toward a target position results in the piezoelectric elements moving the probe asymptotically toward the commanded target position, such that the probe always falls just short of reaching the command target position. Hysteresis means that when the piezoelectric elements are energized to move the probe from a starting position to a target position, the probe advances along a path toward the target position but does not actually return along the same exact path or to the same starting point when the piezoelectric elements are deenergized. Drift means that changes in ambient conditions such as temperature around the probe and sample affect both the motion of the piezoelectric elements as well as the overall position of the probe.
What's worse, in certain SPM applications the exact position of the probe cannot be known with atomic precision, thereby preventing accurate correlation between the spectroscopic analysis and the target location of the surface. For example, certain SPM applications involve extremely harsh environments in which probe position feedback devices are not economically feasible. In other SPM applications below the 10 nm scale, typical probe position feedback devices, such as laser based devices, are not technically feasible.